This invention relates to image intensifier tubes of the type used in night vision viewing systems and, more particularly, to an image intensifier tube with reduced veiling glare and a method of making the same.
Image intensifier tubes multiply the amount of incident light they receive and thus provide an increase in light output which can be supplied either to a camera or directly to the eyes of a viewer. These devices are particularly useful for providing images from dark regions and have both industrial and military application. For example, these devices are used for enhancing the night vision of aviators, for photographing extraterrestial bodies, and for providing night vision to sufferers of retinitis pigmentosa (night blindness).
Modern image intensifier tubes utilize a micro-channel plate (referred to as an MCP) which is a thin glass plate having an array of microscopic holes through it. Each hole is capable of acting as a channel-type secondary emission electron multiplier. When the micro-channel plate is placed in the plane of an electron image in an intensifier tube, one can achieve a gain of up to ten to twenty thousand and extremely high resolution. Since each channel in a micro-channel plate operates nearly independently of all the others, a bright point source of light will saturate a few channels but will not spread out over adjacent areas. This characteristic of "local saturation" makes these tubes more immune to blooming at bright areas. However, these tubes suffer from a problem known as "veiling glare". Veiling glare is the result of scattered light falling on the light input or focal plane of the tube. In the image intensifier it results in a loss of contrast by filling in the darker portions of the image and decreasing the visibility of small or low contrast objects. In fact, in extreme cases it can cause a complete loss of picture information over a substantial part of the field of view.
Veiling glare is due primarily to off-axis light which is reflected into the inside of the tube and is intensified to appear in the field of view as unwanted images. The sources of veiling glare include bright light rays which enter the normal field of view at off-axis angles. The light rays originating from sources outside normal field of view are reflected at the interface of the face plate and the metal conductive layer to the photoemissive material of the tube resulting in emission from the latter to the image focused onto the photoemissive material to cause the unwanted veiling glare.
There have been various attempts to eliminate or reduce the veiling glare by adding material to the tube which absorbs off-axis light and prevents it from being reflected to the photoemissive layer. For example, black rings have been formed on the surface reflecting the off-axis light. These rings have been retained in place by sealing a glass ring to the surface or by fusing a glass ring to the surface to sandwich the ring in between the reflecting surface and the glass ring. This has been difficult to do and is very expensive. Another technique has been to etch a groove between the light input surface and the reflecting surface and to fill the groove with light absorbing material. This too has been difficult to do and is also very expensive. It is, therefore, an object Of the present invention to provide a light image intensifier tube with reduced veiling glare which is economical to make. It is a further object of this invention to provide a method of making such a tube in a highly economical and efficient manner.